Part of the completion process of a prospective oil or gas production well involves cementing the annulus, e.g. the annular space between the well casing and the well bore after a casing string has been run in a wellbore in order to permanently seal the bore and prevent any fluid or gas penetration into the well. Additionally, cementing may be used to seal a lost circulation zone, or an area where there is a reduction or absence of flow within the well. In directional drilling, cementing is used to plug an existing well, in order to run a directional well from that point. Also, cementing is used to plug a well when the well is abandoned. The process of downhole cementing further includes conductor casing and surface casing which are shallow casing strings designed to protect the drilling equipment from the washout and further protect the shallow water zones. Finally, cementing process is utilized in liner casing which is a casing string cemented inside another intermediate casing string but does not extend the full length of the wellbore; it is “hung” at the bottom of a larger casing string and cemented in place.
Cementing is performed when the cement slurry is deployed into the well via pumps. The cement slurry displaces the drilling fluids remaining within the well, and replaces the fluids with cement. Preferably, such displacement should be performed in a continuous manner such that there is no interruption to the flow of cement. Such process requires a continuous delivery of a desired quality and quantity of cement slurry so that the first cement down the hole remains stable until last cement is placed after one continuous operation. Then, the cement must remain stable while it is static up and until the time it sets or hardens. This can take up to 4-6 hours to place cement and up to 12 hours for the cement to set uphole where the temperatures are cooler than the bottom of the well. The cement slurry flows to the bottom of the wellbore through the casing. From there, it fills in the annular space between the casing and the wellbore, and hardens or sets. The hardened cement creates a seal so that outside materials (including gases) cannot enter the well flow, as well as permanently positioning and protecting the casing in place.
Downhole cementing poses particular problems caused by changing temperatures and hydrostatic pressure along the cement column, thus making conventional cement preparation and composition practically unsuitable for a downhole cementing. Preparing slurry having the required physical and chemical properties is essential before commencing cementing operations in a wellbore environment. The goal is to manufacture highly homogeneous, stable, lightweight cement slurry. The proper cement mix is prepared to provide slurry having the required density and viscosity before the slurry is pumped into the hole. Special mixers, including hydraulic jet mixers, re-circulating mixers or batch mixers, are used to combine dry cement with water to create the wet cement or slurry. The cement used in the well cementing process is Portland cement, typically one of the API Classes of Portland cement each employed for different situations and it is typically prepared also with chemical additives to form the final slurry. Additives can include accelerators, which shorten the setting time required for the cement, as well as retarders, which do the opposite and make the cement setting time longer. In order to decrease or increase the density of the cement, lightweight and heavyweight additives are added. The former, for instance, include spheres, bubbles, lightweight minerals such as silica fume. However, the conventional additives used in a lightweight cementing (e.g. glass bubbles or ceramic spheres) are expensive per se, require special bulk mixing procedures to preserve compressive strength of the cement and can be hazardous to the people working closely with them. Additives can be added to transform the compressive strength of the cement, as well as flow properties and dehydration rates. Extenders can be used to expand the cement in an effort to reduce the cost of cementing, and antifoam additives can be added to prevent foaming during mixing and within the well. In order to plug lost circulation zones, bridging materials are added, as well. For setting wells at deep depths, under high temperature or pressure, as well as in corrosive environments, special cements and/or additives are utilized.
Depending upon the particular formation, bore depth, equipment and other factors, it may be necessary or desirable to mix additives with the cement to retard setting, accelerate setting time, control fluid loss in the cement, gel the cement and reduce or increase the slurry density. Additives may be used to increase the mechanical strength of the cement when set, reduce the effect of mud on the cement and to improve the cement bonding. Additives are typically mixed with the cement as the slurry is prepared and before the cement is pumped into the well. In some cases, there may be different pozzolanic materials combined with the cement itself but, as noted above, typically Portland cement is blended with additives and/or modified to accommodate wellbore conditions such as temperatures up to or greater than 600 degrees Fahrenheit. In different variations, binders other than Portland cement may be used, for example, fly ash and other pozzolanic materials.
The physical characteristics of Portland cement and similar binders may tend to create a drag effect, affecting the flow characteristics of the cement. In particular, Portland cement particles have a generally flat shape that creates a drag effect, reducing the flowability of the cement. Adding components such as fly ash in combination with Portland cement can alleviate some of this drag effect, but the addition of fly ash may create other issues such as variability in the heat of hydration of the cement and/or the set time of the cement. Variability in the physical and chemical properties of fly ash utilized as an additive in oilfield cement can also increase variability in the chemistry and physical properties of the slurry. Such variability may be small; however, in well cementing applications, the effect of such variability can be significant.
Water is of course, necessary to hydrate the Portland cement and provide appropriate flow properties. However, if excessive water is used, separation of the water from the cement mixture may occur, especially once the cement stops flowing. Excessive water may also cause loss of strength, excessive shrinkage and variability in hydraulic pressure, which may be detrimental in different applications. Fluid loss additives may be used to reduce a segregation and separation of wellbore cement components and compensate for water imbalance. Fluid loss additives are designed to keep the cement slurry more cohesive over a range of common variables and to mitigate the effect of excessive water in the slurry. However, the increased cohesiveness of the slurry tends to reduce the flow rate of the cement slurry into the bore. One way of improving the flow properties of the cement slurry and/or compensating for the effect of fluid loss products, is to entrain air (or other gas) into the slurry as or before the slurry is injected into the bore. Such air-entrained cement slurry creates a lightweight cement eliminating expensive lightweight admixes such as hollow ceramic or glass microspheres and the costly method of their entrainment.
Also, lightweight cementing with its particular type known also as “foamed cementing” or “cellular cementing” has been utilized in a wellbore cementing to reduce the density of the cement column thereby reducing the hydrostatic pressure of the cement slurry column on the rock formations. Simply, lightweight cements are desirable because they will exert less hydrostatic pressure on the rock formation. If the formation fractures during the cementing process, the cement will enter the formation and compromise the ability to place cement along the entire wellbore; this, ultimately, causes a poor or failed cementing of the wellbore. The light weight cement is of particular usefulness in weak formations, i.e. in formations having relatively low fracture gradient. Foamed cement has several advantages in addition to its low density. It has relatively high compressive strength (which is developed in a reasonable time), causes less damage to water-sensitive formations, can reduce the chance of annular gas flow, and allows cementing past zones experiencing total losses.
Conventional foamed cementing utilizes dispersed gas, usually nitrogen, in cement slurry which contains surfactant as a foaming agent and other chemical products to improve foam stability. The nitrogen is introduced under pressure in a closed system, creating an “energized fluid.” The energized fluid causes several drawbacks in downhole cementing with the utilization of cellular cement. First, since compressed gases are used to produce foamed cement, additional safety precautions are required when performing foamed cement jobs. The gasified cement, as it flows to surface, expands and can produce great force. This can cause it to blow across the pit and erode the pit walls. Also, due to the compressed gases, the fluids have much more potential energy than conventional slurries. The energized fluid in the wellbore requires an additional amount of cement and extra procedural step called cap cement placed into the wellbore down the annulus after the foam cement has been pumped down the casing and up the annulus. The utilization of the cap cement is to add a second mechanism to withhold the pressure of the energized slurry along with a mechanical well control. The second function of the cap cement is to allow a certain amount of pressure to be applied to the annulus to ensure the correct amount of gas compression downhole and assure the proper density of the downhole slurry. The placing of the cap cement incrementally raises the costs of the entire operation. Furthermore, a conductor and surface casing cannot typically be cemented with an energized fluid because there is no well control at surface at this point (no BOP installed) which leaves the annulus open to the air and, therefore, there is no containment for an energized fluid, which will decompress causing a blowout of the well.
During the manufacturing of the foamed cement, conventional air entrainment techniques typically use surfactant formulations which are largely or totally anionic based compositions added to cement slurries. Methods using these compositions are practically unable to compensate for large changes in cement and/or pozzolanic chemistry, agitation conditions, slurry temperatures and other factors that change the characteristics of the slurry. Variations in these parameters limit the capability to predict or calculate slurry yield volume and/or the quality of the air entrainment in the slurry injected into the well. Unstable slurries result in a pore structure which is nonspherical and interconnected. This phenomenon occurs while the cement sets. It is caused by a rupture of unstable nitrogen bubble walls upon contact with other nitrogen bubbles, resulting in coalescence and larger gas pockets. This results in a sponge-like structure with lower compressive strength, higher permeability, and inferior bonding properties. This inability to control sources of variability and/or to compensate for the effects of these sources of variability limits the desirability of utilizing air entrainment (foamed gas) as a method of adjusting slurry parameters or limiting the amount of cement used in the slurry.
Due to a variable hydrostatic pressure and temperatures in a wellbore environment, there is a need to produce cement slurry of variable density to account for a changing hydrostatic pressure in a wellbore column. These real time adjustments in a density of the cement slurry pose a challenge which prior art has yet to solve, for it requires complex pumping schedule with close coordination and control of the treatment on location.
Accordingly, there is a need for safer and more cost-effective downhole cementing system and method that manufactures lightweight, cellular cement in a continuous process but does not use traditionally gasified cement slurry having much higher than desirable potential energy. There is also a need for producing stable and homogeneous cement slurry for a downhole cementing of a wellbore having physical and chemical characteristic which preserve compressive strength at high temperature. There is also further need for well cementing that increases productivity of the well by avoiding the downtime caused by a blow outs due to energized fluid columns, or by abandoning the well in case of a poor/inadequate cementing job. It is a further object of the invention to provide such a system which can be disassembled (has adequate portability) for carrying from place to place, and which can be assembled relatively easily on the site. Thus, there is a need for producing stable, cellular cement slurry with predictable chemical and physical characteristics which assures longevity and safer operation of a producing well.